Impedance matched lane reversal switching system

ABSTRACT

An impedance matched lane reversal switching system including first and second transceiver pairs, each of the pairs including a transmitter connected to a receiver, the output of the transmitter connected to the input of the receiver and to a node, the node of each pair interconnected with a transmission line and a switching circuit for selectively enabling one of the transmitters of one of the transceiver pairs and disabling the other and selectively utilizing one of the receivers of the other of the transceiver pairs and not the other to selectively reverse an egress side and an ingress side of the lane.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/469,869 filed Feb. 3, 2005, incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates generally to a switching system and moreparticularly to an impedance matched lane reversal switching system thatreverses the ingress and egress sides of the lane to provideconnectivity to different types of devices.

BACKGROUND OF THE INVENTION

In high speed switching of digital signals a switching system, typicallypart of a switch card, is connected to a backplane. Multiple line cardsthat each include a plurality of Ethernet connection ports are alsoconnected to the backplane. The Ethernet ports provide connectivity to avast array of digital devices, e.g., computers, printers, and the like,on a typical computer network. The switching system provides high speedswitching of the digital signals to and from the digital devicesconnected to the line cards.

A typical conventional switching system includes, inter alia, atransmitter and a receiver on an IC that are connected to the backplane.The line card, or other similar device, similarly includes a transmitterand receiver connected to a backplane.

A lane includes two logical connections. It includes both the connectionfrom the transmitter of the switching system on the switch card to thebackplane and to the receiver on the line card, and the connection fromthe transmitter on the line card to the backplane and to the receiver onthe switching system on the switch card. A single lane allowstransmitting data from the switch card to the line card and transmittingdata from the line card to the switch card simultaneously. These arecommonly called the ingress (inbound) and egress (outbound) sides of thelane. Data is typically transmitted out to the line card on the egressside and data is received from the line card on the ingress side.Therefore, in order for the line card, or similar device, to functionproperly with the switch card, the egress side of the lane must matchthe receiver on the line card, or similar device, and the ingress sideof the lane must match the transmitter on the line card. Hence, if thereceiver and transmitter of the line card do not match the appropriateegress and ingress sides of the lane the devices may still functionproperly but communication will fail.

Typical prior art lane reversal switching systems that attempt toovercome this problem utilize two ICs that each includes a transmitterand a receiver. The designs utilize one transmitter/receiver pair on onechip connected to the egress and ingress sides of the lane that matchone type of device and utilize the other transmitter/receiver pair onthe other chip that are connected to opposite sides of the lane toprovide connectivity to another type of device that has theconfiguration of its transmitter and receiver reversed.

Because the transmitter and receiver on one of the two chips areconnected to opposite sides of the lane from the transmitter andreceiver on the other chip, two nodes exist at the connection pointbetween the two sides of the lane.

In operation, the DC impedance seen looking into these nodes is lessthan expected, e.g., half the expected impedance. The result of the DCimpedance mismatch is a reduced signal amplitude strength seen at thereceiver.

Associated with each of the transmitters and receivers on the ICs andtheir terminating resistances are bond wires that are connected topackage traces. Outside each IC or chip, card traces connect the packagetraces for the respective transmitters and receivers to a connector thatconnects to the backplane. At high frequency AC, e.g., 3.2 Gbits/sec,the transition time for a pulse is approximately 100 picoseconds, whichapproaches the travel time of the pulse through the package traces andcard traces. At such high frequencies the card traces and package tracesbehave like transmission lines and have a characteristic impedanceassociated with them. Therefore, the high frequency AC impedance seenlooking into the two nodes on the two sides of the lane is less than theexpected high frequency AC impedance. The result of this high frequencyAC impedance mismatch is reflections at two nodes. When the designincludes a terminating resistance connected to each transmitter and eachreceiver of the transmitter/receiver pairs, then a reduced highfrequency signal amplitude is received by the active receiver. If thedesign eliminates the terminating resistances connected to the receiversof the transmitter/receiver pairs on the ICs to provide DC impedancematching, then the high frequency impedance mismatch results inreflections not only at the nodes on the sides of the lane, but also atthe receivers of the transmitter/receiver pairs on the ICs. Theseadditional high frequency reflections at the receivers cause pulse edgedistortion.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide an impedancematched lane reversal switching system.

It is a further object of this invention to provide such a system whichreduces high frequency reflections.

It is a further object of this invention to provide such a system whichimproves high frequency impedance matching.

It is a further object of this invention to provide such a system whichreduces pulse edge distortion.

It is a further object of this invention to provide such a system whichcan be integrated on a single chip.

It is a further object of this invention to provide such a system whichrequires only a single terminating resistance for each transmitter andreceiver pair.

This invention results from the realization that an impedance matchedlane reversal switching system that provides connectivity to devicesthat have different orientation of their transmitters and receivers andprovides both DC and high frequency AC impedance matching can beeffected on a single chip by utilizing a pair of transceivers that eachinclude a transmitter connected to a receiver wherein the output of thetransmitter is connected to the input of the receiver and to a node andeach node is connected to a transmission line, and a switching circuitthat selectively enables one of the transmitters of one of thetransceiver pairs and disables the other and utilizes one of thereceivers of the other transceiver pair and not the other to selectivelyreverse the egress and ingress side of a lane so that devices that haveopposite orientations of their transmitters and receivers can beutilized. This invention results from the further realization thatutilizing a single chip and a single terminating resistance for eachtransmitter and receiver pair eliminates nodes on the lane and provideshigh frequency AC impedance matching and virtually eliminatesreflections and pulse edge distortion.

The subject invention, however, in other embodiments, need not achieveall these objectives and the claims hereof should not be limited tostructures or methods capable of achieving these objectives.

This invention features an impedance matched lane reversal switchingsystem including first and second transceiver pairs, each of the pairsincluding a transmitter connected to a receiver, the output of thetransmitter connected to the input of the receiver and to a node, thenode of each pair interconnected with a transmission line, and aswitching circuit for selectively enabling one of the transmitters ofone of the transceiver pairs and disabling the other and selectivelyutilizing one of the receivers of the other of the transceiver pairs andnot the other to selectively reverse an egress side and an ingress sideof the lane.

In one embodiment, the system may include a set of terminatingresistances interconnected to each node and to each transmission linefor impedance matching and terminating both the transmitter and receiverof each of the transceiver pairs. The system may include a connection toat least two types of devices that have different orientation of theirtransmitters and receivers. The at least two types of devices mayinclude line cards or switch cards. The system may be integrated on asingle chip. The single chip may be disposed on a switch card. Thesingle chip may be disposed on a line card. The switching circuit mayinclude a plurality of switching devices for selectively enabling one ofthe transmitters of one of the transceiver pairs and utilizing one ofthe receivers of the other transceiver pairs in response to a controlsignal. The switching circuit may include an external control pin. Theswitching circuit may include a cross bar circuit having at least oneinput and at least one output for selectively connecting the at leastone output to an enabled one of the transmitters of the transceiverpairs and the at least one input to a utilized one of the receivers ofthe other transceiver pair.

This invention also features an impedance matched lane reversalswitching system including first and second transceiver pairs, each ofthe pairs including a transmitter connected to a receiver, the output ofthe transmitter connected to the input of the receiver and to a node,the node of each pair interconnected with a transmission line, and aswitching circuit for selectively enabling one of the transmitters ofthe transceiver pairs and disabling the other in one mode to selectivelyreverse an egress side and an ingress side of the lane.

This invention also features a method of impedance matching and lanereversing a switching system including the steps of providing first andsecond transceiver pairs, each of the pairs including a transmitterconnected to a receiver, the output of the transmitter connected to theinput of the receiver and to a node, the node of each pairinterconnected with a transmission line, and selectively enabling one ofthe transmitters of one of the transceiver pairs and disabling the otherand selectively utilizing one of the receivers of the other of thetransceiver pairs and not the other to selectively reverse an egressside and an ingress side of the lane.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled inthe art from the following description of a preferred embodiment and theaccompanying drawings, in which:

FIG. 1 is a schematic block diagram of a prior art lane reversalswitching system;

FIG. 2 is a graph showing a waveform with a reduced signal outputgenerated by the lane reversal switching system shown in FIG. 1;

FIG. 3 is a schematic block diagram of another prior art lane reversalswitching system;

FIG. 4 is a graph showing a waveform with pulse edge distortiongenerated by the lane reversal switching system shown in FIG. 3;

FIG. 5 is a schematic block diagram of yet another prior art lanereversal switching system;

FIG. 6 is a schematic block diagram of one embodiment of the impedancematched lane reversal switching system of this invention; and

FIG. 7 is schematic block diagram of another embodiment of the impedancematched lane reversal switching system of this invention.

PREFERRED EMBODIMENT

Aside from the preferred embodiment or embodiments disclosed below, thisinvention is capable of other embodiments and of being practiced orbeing carried out in various ways. Thus, it is to be understood that theinvention is not limited in its application to the details ofconstruction and the arrangements of components set forth in thefollowing description or illustrated in the drawings. If only oneembodiment is described herein, the claims hereof are not to be limitedto that embodiment. Moreover, the claims hereof are not to be readrestrictively unless there is clear and convincing evidence manifestinga certain exclusion, restriction, or disclaimer.

Conventional prior art lane reversal switching system 10, FIG. 1includes IC 12 and IC 14 that are employed in typical switch card 15. IC12 includes transmitter 16 with terminating resistance 26, bond wire 66and package trace 68 connected to egress (outbound) side 20 of lane 22and receiver 18 with terminating resistance 28, bond wire 72 and packagetrace 74 connected to ingress (inbound) side 24 of lane 22. Egress side20 and ingress side 24 connect to backplane transmission lines 33 and 35of backplane 32 via connector 30. Device 34, e.g., a line card, orsimilar device connects to backplane 32 via connector 37. In thisexample, device 34 includes receiver 38 with terminating resistance 58and card trace 39 aligned with egress side 20 and transmitter 36 withterminating resistance 56 and card trace 41 aligned with ingress side24.

In operation, data is transmitted by transmitter 16 on IC 12 to receiver38 on device 34 via egress (outbound) side 20 of lane 22, indicated byarrow 21, and data is received by receiver 18 on IC 12 from transmitter36 on device 34 via ingress (inbound) side 24 of lane 22, indicated byarrow 25. The line card (device 34) may also include an IC havingreceivers 38 and 38′ and transmitters 36 and 36′. However, if theorientation of transmitter 36 and receiver 38 in device 34 is reversed,e.g., transmitter 36′ is aligned with egress side 20 of lane 22 andreceiver 38′ is aligned with ingress side 24 of lane 22, communicationto device 34 will fail. To accommodate for this situation, conventionalsystem 10 includes IC 14 that includes transmitter 48 and receiver 50which are connected to opposite sides of lane 22, e.g., transmitter 48is connected to side 24 of lane 22 and receiver 50 is connected to side20. Hence, side 24 of lane 22 now acts as the egress (outbound) side oflane 22, indicated by arrow 49 (shown in phantom) and data istransmitted from transmitter 48 on IC 14 to receiver 38′. Similarly,side 20 now acts the ingress (inbound) side of lane 22, indicated byarrow 51 (shown in phantom) and data transmitted by transmitter 36′ isreceived by receiver 50 on IC 14. Hence, system 10 has reversed theingress and egress sides of lane 22 to match the configuration oftransmitter 36′ and receiver 38′ in device 34.

However, the connection between transmitter 48 on IC 14 to side 24 oflane 22 and the connection between receiver 50 to side 20 of lane 22results in nodes 62 and 64, respectively. The result is that the DCimpedance seen at nodes 62 and 64 is less than the desired impedanceassociated with the terminating resistance of the active transmitter. Inthis example, the impedance at node 64 is half the expected impedance ofterminating resistance 26 when transmitter 16 is enabled due toterminating resistances 54 and 58 and the impedance at node 62 is halfof the expected impedance of terminating resistance 56 when transmitter36 is active due to terminating resistances 28 and 52. The result is areduced DC signal amplitude being received at receiver 38 whentransmitter 16 is active and at receiver 50 when transmitter 36 isactive. The same mismatched impedance is seen at nodes 62 and 64 whentransmitter 48 is active and transmitter 36′ is active.

The high frequency AC impedance seen at node 64 will also be less thanthat expected when transmitter 16 is enabled due to the card trace 88and package trace 86 behaving like transmission lines with acharacteristic impedance. The high frequency impedance seen at node 62is also less than expected when transmitter 36 is active and receiver 18is used due to card trace 76 and package trace 80 behaving liketransmission lines. The same high frequency impedance mismatch is foundat node 62 when transmitter 48 is active and at node 64 when transmitter36′ is active and receiver 50 is used.

The high frequency impedance mismatch at node 64 results in the onethird (⅓) of the signal on egress side 20 of lane 22 (when transmitter16 is active) being reflected back to transmitter 16 where the signalends due to terminating resistance 26. A fraction of the signal travelsthrough node 64 to receiver 38 with terminating resistance 58 where thesignal ends. The result is the signal received by receiver 38 has areduced signal amplitude, e.g., two thirds (⅔) the expected value.Similarly, the signal received by receiver 18 when transmitter 36 isactive will also be reduced.

FIG. 2 shows an example of the signal 90 that has a reduced signalamplitude when compared to desired receiver signal 92 received by activereceivers discussed above.

Another conventional prior art lane reversal switching system 10′, FIG.3, where like parts have been given like numbers, utilizes a designsimilar to system 10 described above except terminating resistance 26associated with transmitter 16 is located off IC 12 and terminatingresistance 52 associated with transmitter 48 is located off IC 14. Thedesign also removes the terminating resistances associated with receiver18 on IC 12 and receiver 50 on IC 14.

One improvement to this design is that the DC impedance seen at nodes 62and 64 matches the terminating resistance associated with activetransmitter.

However, the high frequency AC impedance at nodes 62 and 64 remainsmismatched due to the various card traces 70, 88, 76, and 82 and packagetraces 80 and 86 behaving like transmission lines with a characteristicimpedance at the nodes. The high frequency AC impedance at node 62 isfurther decreased due to terminating resistance 52.

Similarly, the high frequency impedance mismatch at nodes 62 and 64causes a portion of the signal to be reflected back to the activetransmitter and a fraction of the signal to travel through the node.However, because there are no terminating resistances on receiver 18 onIC 12 and receiver 50 on IC 14, the fraction of the signal that travelsthrough nodes 62 and 64 will be reflected back to the correspondingactive transmitter. These reflections result in pulse edge distortion ofthe signal received by receiver 38.

Waveform 94, FIG. 4, shows an example of the pulse edge distortion thatresults from the high frequency impedance mismatch at nodes 62 and 64discussed above. Waveform 96 shows an example of a desired signal thatwould be received by the active receiver.

Prior art lane reversal switching system 10″, FIG. 5 where like partshave been given like numbers, similarly uses a single terminatingresistance for each transmitter on IC 12 and IC 14. This design movesterminating resistance 26 that was previously located off IC 12, asshown by arrow 99, FIG. 3 to on IC 12 as shown by arrow 101, FIG. 5.Similarly, terminating resistance 52 is moved on IC 14, as shown byarrow 103. Although this design also improves DC impedance matching atnodes 62 and 64, the design also suffers from high frequency impedancemismatching at nodes 62 and 64 due to the various package traces andcard traces that behave like transmission lines with a characteristicimpedance. The design does remove three points of reflection at e.g., atnodes 67, 69, and 71, FIG. 3, because terminating resistances 26 and 52,FIG. 5, are moved on chip.

In contrast, impedance matched lane reversal switching system 150, FIG.6 of this invention, effectively reverses the ingress and egress sidesof the lane to provide connectivity to a plurality of devices, e.g.,line cards, switch cards, and the like, that have differentconfigurations of their transmitters and receivers. System 150 alsoprovides both DC and high frequency AC' impedance matching byeliminating nodes on the sides of the lane and using a singleterminating resistance connected to each transmitter and receiver pair.The result is that reflections are virtually eliminated and a fullstrength signal is received by the active receiver that is virtuallyfree of pulse edge distortion.

Impedance matched lane reversal switching system 150 includes firsttransceiver pair 152 and second transceiver pair 154. Transceiver pair152 includes transmitter 156 and receiver 158. The output of transmitter156 connects to the input of receiver 158 by line 160 and to node 162.Terminating resistance 164 is attached to node 162. Node 162 connects tobond wire 163 that attaches to package trace 165. Package trace 165interconnects to card trace 206. Transceiver pair 154 includes receiver168 and transmitter 170. The output of transmitter 170 connects to theinput of receiver 168 by line 172 and to node 174. Terminatingresistance 176 is attached to node 174. Node 174 attaches to bond wire177 that connects to package trace 208. Package trace 208 interconnectsto card trace 210. Preferably, transceiver pairs 152 and 154 withterminating resistances 164 and 176, respectively, are on a single chip,e.g., chip 250 that is included on a typical switch card, e.g., switchcard 252.

Card traces 206 and 210 affix to connector 211 that couple to backplanetransmission lines 204 and 212 of backplane 213. Connector 215interconnects backplane 213 to device 200, e.g., a line card, switchcard, or similar device. Device 200 typically includes receiver 184 withterminating resistance 216 and transmitter 186 with terminatingresistance 188 interconnected to connector 215 via package traces 202and 214, respectively.

As described above, package traces 165 and 208, and card traces 206 and210 on switch card 252, as well as package traces 202 and 214 on device200 behave like transmission lines with a characteristic impedance(e.g., 50 Ω) at high frequencies (e.g., 3.2 Gbits/sec) when thetransition time of a pulse approaches the travel time of the pulsethrough the package traces and card traces.

Switching circuit 185 selectively enables one of transmitters 156 or 170of transceiver pairs 152 and 154 and disables the other and utilizes oneof receivers 158 or 168 of the opposite transceiver pairs 152 and 154 ofthe enabled transmitter 156 or 170 to selectively reverse sides 180 and182 of lane 198.

For example, switching circuit 185 may selectively enable transmitter156 of transceiver pair 152, disable transmitter 170 of transceiver pair154, utilize receiver 168 of transceiver pair 154 and not utilizereceiver 158 of transceiver pair 152 to provide connectivity to device200 with receiver 184 and transmitter 186. In this example, data istransmitted by transmitter 156 of transceiver pair 152 on egress side180 of lane 198, indicated by arrow 181, to receiver 184 on device 200.Data is transmitted by transmitter 186 on device 200 to receiver 168 oftransceiver pair 154 on ingress side 182 of lane 198, indicated by arrow183.

To provide connectivity to device 200, e.g., a line card, another switchcard, or similar device with transmitter 186′ and receiver 184′, (shownin phantom) that are in opposite configuration as transmitter 186 andreceiver 184, switching circuit 185 enables transmitter 170 oftransceiver pair 154 and disables transmitter 156 of transceiver pair152 and utilizes receiver 158 of transceiver pair 152 and does notutilize receiver 168 of transceiver pair 154. In this way, data will betransmitted by transmitter 170 of transceiver pair 154 on side 182 oflane 198, which now acts as the egress side of the lane, indicated byarrow 187 (shown in phantom) to receiver 184′. Similarly, data will betransmitted by transmitter 186′ on device 200 on side 180 of lane 198,which now acts as the ingress side of the lane, indicated by arrow 189(shown in phantom) to receiver 158 of transceiver pair 152. The resultis that switching circuit 185 of system 150 has effectively reversed theingress and egress sides of lane 198 to provide connectivity to device200 that has different orientation of its transmitters and receivers.Because lane reversal switching system 150 is on a single chip, e.g.,chip 250, and eliminates the nodes on the sides of the lanes as found inthe prior art, system 150 provides both DC and high frequency ACimpedance matching.

The DC impedance is matched because the DC impedance of the terminatingresistance of the active transmitter matches the terminating resistanceof the utilized receiver. For example, terminating resistance 164, e.g.,50 Ω, associated with active transmitter 156, matches terminatingresistance 216, e.g., 50 Ω, associated with active receiver 184 ondevice 200. Similarly, terminating resistance 188 associated with activetransmitter 186 matches terminating resistance 176 associated withactive receiver 168. The same DC impedance matching is found whentransmitter 170 is active and receiver 184′ is used and when transmitter186′ is enabled and receiver 158 is used.

The high frequency AC impedance associated with the terminatingresistance of the active transmitter is matched with the characteristicimpedance associated with the various package traces 165 and 208, cardtraces 206 and 210, and package traces 202 and 214 that behave liketransmission lines with a characteristic impedance, as well as backplanetransmission lines 204 and 212.

For example, if transmitter 156 of transceiver pair 152 is enabled andterminating resistance 164 is, e.g., 50 Ω, and package trace 165 behaveslike a transmission line with a characteristic impedance, e.g., 50 Ω,the signal traveling from transmitter 156 to package trace 165 will meeta matched high frequency impedance and no reflection will occur. Whenthe signal travels across card trace 206, backplane transmission line204, and package trace 202, all of which behave like a transmission linewith a characteristic impedance, e.g., 50 Ω, no reflection will occur.When the signal meets terminating resistance 216, e.g., 50 Ω, associatedwith active receiver 184, no reflection will result and the signal thatis received by receiver 184 is a full strength signal. Similarly, whentransmitter 186 is active and receiver 168 is used, no reflection willoccur. The same result is also found when transmitter 170 is enabled andreceiver 158 is used. Because receiver 158 of transceiver pair 152 isassociated with terminating resistance 164, e.g., 50 Ω, and receiver 168of transceiver pair 154 is associated with terminating resistance 176,e.g., 50 Ω, no reflection will occur at active receiver 158 or 168. Theresult is that high frequency reflections are virtually eliminated as isthe pulse edge distortion associated therewith.

Switching circuit 185 is responsive to an external control signal fromexternal pin 191 by line 192 that selectively switches switching device194 and switching device 196 to nodes 197 and 199, respectively, toenable transmitter 156 of transceiver pair 152 and utilize receiver 168of transceiver pair 154. In this example, the control signal on line 192may be a logical high (e.g. CTL=1) that enables transmitter 156. Thecontrol signal on line 192 also switches switching device 194 andswitching device 196 to nodes 201 and 203, respectively, to enabletransmitter 170 of transceiver pair 154 and to utilize receiver 158 oftransceiver pair 152. In this example, the externally generated controlsignal on line 192 may be a logic low (e.g., CTL=0). The output ofinverter 195 is connected to transmitter 170 which enables transmitter170 when CTL=0 ( CTL=1).

Switching circuit 185 also includes cross bar circuit 260, FIG. 7,having at least one input and at least one output, e.g., receiver output262 and transmitter input 264. Cross bar circuit 260 selectivelyconnects receiver output 262 to one of the transmitters of transceiverpairs 152 and 154 and connects transmitter input 264 to one of thereceivers of the other of transceiver pairs 152 and 154. For example,cross bar circuit 260 may connect receiver output 262 to transmitter 170of transceiver pair 154 and transmitter input 264 to receiver 158 oftransceiver pair 152, e.g., when the control signal is low (CTL=0).Cross bar circuit 260 may also connect receive output 262 to transmitter156 of transceiver pair 152 and transmitter input 264 to receiver 168 oftransceiver pair 154, e.g., when the control signal is high (CTL=1).Similarly, cross bar circuit 260 may also connect receiver output 266and transmitter input 268 to transmitter 170 and receiver 158,respectively, as well as to transmitter 156 and receiver 168.

As described above, system 150 effectively reverses sides 180 and 182 oflane 198 to provide connectivity to a device, e.g., a line card orswitch card that has opposite orientation of its transmitters andreceivers, such as devices 200 and 200′.

Although specific features of the invention are shown in some drawingsand not in others, this is for convenience only as each feature may becombined with any or all of the other features in accordance with theinvention. The words “including”, “comprising”, “having”, and “with” asused herein are to be interpreted broadly and comprehensively and arenot limited to any physical interconnection. Moreover, any embodimentsdisclosed in the subject application are not to be taken as the onlypossible embodiments. Other embodiments will occur to those skilled inthe art and are within the following claims.

In addition, any amendment presented during the prosecution of thepatent application for this patent is not a disclaimer of any claimelement presented in the application as filed: those skilled in the artcannot reasonably be expected to draft a claim that would literallyencompass all possible equivalents, many equivalents will beunforeseeable at the time of the amendment and are beyond a fairinterpretation of what is to be surrendered (if anything), the rationaleunderlying the amendment may bear no more than a tangential relation tomany equivalents, and/or there are many other reasons the applicant cannot be expected to describe certain insubstantial substitutes for anyclaim element amended.

1. An impedance matched lane reversal switching system comprising: firstand second transceiver pairs, each of said pairs including a transmitterconnected to a receiver, the output of the transmitter connected to theinput of the receiver and to a node, the node of each pairinterconnected with a transmission line; and a switching circuit forselectively enabling one of the transmitters of one of said transceiverpairs and disabling the other and selectively utilizing one of thereceivers of the other of said transceiver pairs and not the other toselectively reverse an egress side and an ingress side of the lane. 2.The system of claim 1 further including a set of terminating resistancesinterconnected to each said node and to each said transmission line forimpedance matching and terminating both the transmitter and receiver ofeach of said transceiver pairs.
 3. The system of claim 1 furtherincluding a connection to at least two types of devices that havedifferent orientation of their transmitters and receivers.
 4. The systemof claim 3 in which said at least two types of devices include linecards.
 5. The system of claim 3 in which said at least two types ofdevices include switch cards.
 6. The system of claim 1 in which saidsystem is integrated on a single chip.
 7. The system of claim 6 in whichsaid single chip is disposed on a switch card.
 8. The system of claim 6in which said single chip is disposed on a line card.
 9. The system ofclaim 1 in which said switching circuit includes a plurality ofswitching devices for selectively enabling one of said transmitters ofone of said transceiver pairs and utilizing one of said receivers of theother said transceiver pairs in response to an external control signal.10. The system of claim 9 in which said switching circuit includes anexternal control pin.
 11. The system of claim 1 in which the switchingcircuit includes a cross bar circuit having at least one input and atleast one output for selectively connecting said at least one output toan enabled one of said transmitters of said transceiver pairs and saidat least one input to a utilized one of the receivers of the othertransceiver pair.
 12. An impedance matched lane reversal switchingsystem comprising: first and second transceiver pairs, each of saidpairs including a transmitter connected to a receiver, the output of thetransmitter connected to the input of the receiver and to a node, thenode of each pair interconnected with a transmission line; and aswitching circuit for selectively enabling one of the transmitters ofsaid transceiver pairs and disabling the other in one mode toselectively reverse an egress side and an ingress side of the lane. 13.A method of impedance matching and lane reversing a switching systemcomprising the steps of: providing first and second transceiver pairs,each of said pairs including a transmitter connected to a receiver, theoutput of the transmitter connected to the input of the receiver and toa node, the node of each pair interconnected with a transmission line;and selectively enabling one of the transmitters of one of saidtransceiver pairs and disabling the other and selectively utilizing oneof the receivers of the other of said transceiver pairs and not theother to selectively reverse an egress side and an ingress side of thelane.